Assessment of vascular physiology may be important in detecting and tracking atherosclerosis and it is also important in head failure, hypertension and diabetes research. Vascular physiology can be assessed, in part, through measurements of endothelial function and arterial compliance. Endothelial function can be assessed by measuring changes in the diameter of an artery in response to a stimulus such as change in blood flow velocity through the artery (more properly, a change in arterial wall shear stress). Endothelial function can be assessed by inflating a blood pressure cuff around a subject's forearm and monitoring velocity of blood flowing through a brachial artery while measuring the artery's diameter before, during and after the inflation of the cuff. Arterial compliance is a measure of change in diameter of an artery in response to a change in blood pressure within the artery.
Ultrasonic images commonly provide a basis for assessing vascular physiology. An ultrasonic imager utilizes a transducer to project an ultrasonic beam into a subject and receive echoes reflected from various anatomic features located within the subject. By timing the echoes, the imager calculates depths of the features and renders an image of the features on a display screen. An operator can then ascertain arterial diameter by measuring distances on the screen, such as a distance between "near" and "far" walls of an artery.
Ultrasonic imaging utilizes a finite "imaging plane" to display an image of a cross-section of a subject's anatomy. An ultrasonic imager constructs the image from echoes from features that the imaging plane intersects. An operator orients a transducer and, thereby, orients its associated imaging plane. The orientation of the imaging plane relative to an artery determines how the artery is represented in an image. For example, if a cross-sectional representation of an artery is desired, the operator orients the imaging plane perpendicular to the artery (herein referred to as "transverse imaging").
Two factors determine accuracy of arterial diameter estimates from "B-mode" images: how an image of an artery is acquired and how the diameter is calculated from the image. In "B-mode" ultrasonic images, brightness of a displayed feature is proportional to strength of echoes from the feature. Image acquisition depends on the skill of an operator and which representation (transverse, longitudinal, etc. ) of the artery is desired. In the prior art, an operator attempts to orient an imaging plane parallel to, and passing through the centerline of, an artery. This orientation (herein referred to as "longitudinal") represents the near and far walls as two parallel lines. The operator estimates diameter of the artery by measuring the distance between the two parallel lines. Specifically, the operator selects a point on each line and the imager calculates distance between the two points. The two points are commonly known as an "ultrasonic caliper." Problematically, diameter estimates made with ultrasonic calipers are suboptimal because they use only a fraction of available arterial wall edge information, and edge detection by the operator is subjective.
Longitudinal imaging of an artery generally provides more arterial wall information than transverse imaging because more of the imaging plane intersects relatively perpendicular features of the artery. The near and far walls are represented by lines in a longitudinal image but only by a small number of points in a transverse image. The lines in a longitudinal image thus provide more points on a display from which an operator can estimate diameter.
Problematically, diameter estimates of an artery made from longitudinal imaging planes tend to be underestimates due to deviation of the imaging plane from the centerline of the artery. In the prior art, an operator attempts to avoid underestimation by repeatedly reorienting the transducer, thereby reorienting the imaging plane, while searching for a combination of: 1) a largest representation of the artery, 2) a strongest set of echoes from near and far walls, thereby signifying perpendicular beam incidents, and 3) a best "7-zone" representation. An artery wall comprises several layers or "zones," which can appear in ultrasonic images of the artery. The "7-zone" appearance can only be obtained (in theory) when a longitudinal imaging plane passes through the centerline of an artery. Locating an imaging plane with a best 7-zone representation is difficult and requires subtle operator skill. Accordingly, an operator cannot assess accuracy of an estimate because he cannot prove that he acquired an image through the centerline of an artery.
Despite employing this three-pad strategy to avoid underestimation, errors in diameter estimates are likely because longitudinal imaging is sensitive to lateral movement of an artery relative to the transducer. Arteries commonly displace relatively large distances in response to stimuli (e.g. application of a blood pressure cuff) used during endothelial reactivity measurements. Lateral movement of an artery a distance of only 10% of an arterial diameter (0.4 mm for a medium-sized 4 mm vessel) relative to the imaging plane results in a 2% error in the diameter measurement.
It is, therefore, an objective of the present invention to provide increased accuracy in ultrasonic diameter estimates of an artery. It is a further objective to provide accurate arterial diameter measurements from images that are temporally spaced, i.e. to provide estimates that are insensitive to lateral movements of a transducer, subject or artery between the first and subsequent images. It is a yet further objective to enable an operator to ascertain whether an arterial diameter estimate is inaccurate due to mispositioning of an imaging plane relative to an artery.